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      这里设置内边距的值为 1em。浏览器将其乘以字号，最终渲染为 16px。这一点很重要：浏览器会根据相对单位的值计算出绝对值，称作计算值（computed value）。

      123456789784567894654651
      
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
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      <p>1</p>
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      <p>1</p>
      <p>1</p>
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      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
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      <p>1</p>
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      <p>1</p>
      <p>1</p>
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      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
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      <p>1</p>
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      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
      <p>1</p>
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